i.mx: introduce the armv7/imx-common folder

In order to support the coming MX6 platform and to reducde
the duplicated code, we had better move some common files
or functions to the imx-common folder for sharing.

This patch does the following:
- move speed.c file from armv7/mx5/speed.c to armv7/imx-common/speed.c
- move armv7/mx5/timer.c to armv7/imx-common/timer.c, no any new feature
  added but just fix the checkpatch errors in the old file and remove
  the CONFIG_SYS_MX5_CLK32 reference in the file
- create one new file cpu.c file to store the common function with i.mx5/6

Signed-off-by: Jason Liu <jason.hui@linaro.org>
Cc:Stefano Babic <sbabic@denx.de>
Acked-by: Stefano Babic <sbabic@denx.de>